Problem: Which of these numbers is prime? ${39,\ 55,\ 61,\ 88,\ 95}$
Explanation: A prime number has exactly two factors: 1 and itself. The factors of 39 are 1, 3, 13, and 39. The factors of 55 are 1, 5, 11, and 55. The factors of 61 are 1 and 61. The factors of 88 are 1, 2, 4, 8, 11, 22, 44, and 88. The factors of 95 are 1, 5, 19, and 95. Thus, 61 is a prime number.